1. Field of the Invention
The present invention relates to a nitride single crystal and a producing method thereof, and more particularly to a nitride single crystal and a producing method of the nitride single crystal employing liquid-phase sintering for its growth.
2. Description of the Background Art
Nitride semiconductors such as AlN and Si3N4 have attracted attention for use as light emitting devices emitting visible or ultraviolet light, and as electronic devices operating at high voltage and large current. Although a material such as sapphire or silicon carbide has been employed for a substrate for producing such an electronic device, it is difficult to form a favorable epitaxial wafer due to large lattice mismatch and a large difference in thermal expansion coefficient between the material and the nitride semiconductor. As such, a nitride substrate ensuring lattice match with the nitride semiconductor has been desired for improvement of properties of the electronic device.
Normally, sublimation, vapor-phase growth or melting is used for growth of a single crystal. It however is difficult to grow a single crystal of nitride such as AlN or Si3N4 by melting, since the nitride has a low decomposition temperature and requires a high temperature and a high pressure to increase the decomposition temperature higher than a melting temperature. As such, vapor-phase growth or sublimation is used to grow a single crystal of the nitride.
In vapor-phase growth, AlN (g) is generated from the reaction of 2Al (s)+2NH3 (g)→2AlN (g)+3H2 (g), and is precipitated as a single crystal. It however is difficult to obtain a large single crystal, since Al as the raw material is highly reactive, making it difficult to control the reaction.
Further, in sublimation, for example, aluminum nitride is sublimated and decomposed and then recombined and recrystallized to obtain a single crystal. With this method, again, it is difficult to obtain a large single crystal, since the crystal growth rate at the time of recrystallization is slow (see the article by Motoyuki Tanaka, titled “Growth of AlN Single Crystal by the Sublimation Method”, Journal of Japanese Association for Crystal Growth, Japanese Association for Crystal Growth, 1998, Vol. 25, No. 4, pp. 163-166).
On the other hand, in fabrication of polycrystalline aluminum nitride widely used as a heat dissipation substrate for an electronic component, liquid-phase sintering is used to make a small crystal grow larger to thereby improve properties, such as thermal conductivity and others, of the product